.
He goes on to discuss two mechanisms that might account for the breakup of ice sheets or outlet glaciers leading to a speedup of the upstream ice sheet. To better understand what we are discussing lets consider the images below:
In the three images we can observe the breakup of a large piece of the ice shelf flowing out of the Petermann glacier. One can clearly see the ice shelf arm which is floating in the ocean and which is connected to the glacier proper behind, which is anchored to the land. Over a relatively short time a large part of the ice sheet breaks up as can be seen in the photos. The result of this breakup, as it turns out is, in most instances, to cause the ice sheet behind the ice shelf to move more rapidly. This has a consequence for sea-level rise, because the ice sheet is on land whereas the ice shelf, as you will remember was floating although anchored in places to the sea bed.
It had generally been assumed for a number of years now that the reason that the movement of ice sheets increased was due to warming at the glaciers surface leading to the creation of meltwater which worked its way into the glacier itself, creating what are know as moulins, holes in the glacier that eventually allow the meltwater to reach the glacier’s base thus ” greasing it” and speeding up its movement. This is known as the Zwally effect What Pelto explains is that although the Zwally effect is real, it is not capable of explaining large increases in ice sheet movement that may continue for many years.
Photo above right: Meltwater stream flowing into a large moulin in the ablation zone (area below the equilibrium line) of the Greenland ice sheet. (Image courtesy Roger J. Braithwaite, The University of Manchester, UK via GISS)
So, if the Zwally effect cannot account for the sudden, rapid movement of ice sheets (up to 20 kilometers per year), what can.
Pelto explains:
The second mechanism is a dynamic thinning of the terminus zone of the marine terminating outlet glacier reducing the effective bed pressure, allowing acceleration – the Jakobshavn effect. The reduced resistive force at the calving front due to the thinner ice, now experiencing greater flotation, is then propagated “up glacier” (Hughes, 1986; Thomas, 2003 and 2004). If the Jakobshavn effect is the key the velocity increase will propagate up-glacier, there will be no seasonal cycle, and thinning and acceleration would be greatest near the terminus.
In other words, the glacier thins where the ice shelf ends, at the place where it calves icebergs into the sea. As the ice shelf thins it becomes a less effective cork for the ice sheet behind it. If it breaks up dramatically as happened with the Wilkins ice sheet in Antarctica this austral winter (!) it may produce rapid movement in the glacier or ice sheet behind it because the remaining ice shelf is simply not strong enough to hold the upstream glacier or ice sheet back.
A series of Landsat images [of the Petermann glacier] from 2002, 2006 and 2007 illustrate the shift in the terminus and in the position of key rifts A, B and C. The distance back from the terminus has diminished for A and B from 2002 to 2007. In 2006 to 2007 the shift in the position of C is also evident.
But why does the ice shelf thin at its terminus? Pelto goes on:
The reasons for Ice Shelf collapse continue to be identified, but one key thread emerges. The decade prior to collapse the Larsen-B Ice Shelf had thinned primarily by melting of the ice shelf bottom (by the ocean) by 18 m (Shepard and others, 2003). Thinning preconditions the ice shelf for failure by weakening its connection to pinning points at the grounding line as the shelf becomes more buoyant. Glasser and Scambos (2008) observed that prior to collapse that rifts and crevasses parallel to the ice front crosscut the meltwater channels and ponds, hence, post dated them. The number and length of the rifts increased markedly in the year before collapse. There was no evidence of relict rifts, illustrating that these rifts are a feature of the last 20 years. After ice shelf collapse the ice front receded to the pre-existing rifts, and the pre-existing rifts defined the area of collapse. In this case the thinning and resultant structural weaknesses preconditioned the ice to rapid breakup, which proceeded to lose only the preconditioned portion of the ice shelf.
and concludes:
It appears then that glacier or ice shelf thinning is the key preconditioning factor for collapse, retreat and acceleration, whether you are in Antarctica of Greenland. The mechanisms for ice shelf thinning include basal melting (from warming ocean waters), surface melting, reduction in glacier inflow and rift development. These are interrelated mechanisms that precondition the ice shelves to collapse. On marine terminating outlet glaciers the mechanisms to trigger thinning is surface ablation causing thinning, and potentially basal melting, though not yet observed (though see this recent paper by Holland et al, 2008). Once the process begins thinner less buttressed ice enables acceleration and more calving and more retreat. There is a potential difference between the two, in glacier such as most marine terminating outlet glaciers, where the glacier flow is rapid, acceleration results from retreat and thinning. In the case of ice shelves a glacier buttressed by them will accelerate after the loss, but the slow moving ice shelf may suffer from reduced inflow. Attention will continue to be focused on these rapid responders to climate change;marine terminating glaciers in Greenland and ice shelves in Antarctica. We can look forward to more details from the extensive 2008 summer field season in Greenland and the upcoming view of the Wilkins this fall.
There is much more detail in Pelto’s fascinating Real Climate post. If you’re interested in such things I highly recommend it.
http://www.dailykos.com/storyonly/2008/11/6/14484/7786/58/655917
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